Introduction
Global climate change is causing an increment in the frequency of
extreme weather events (Stocker et al. 2014) that are recognized
among the major drivers of current and future ecosystem dynamics (Franket al. 2015). The Mediterranean basin is one of the two main
hot-spots of climate change (Giorgi 2006; Noce, Collalti & Santini
2017), showing increases in the inter annual variability and of extreme
environmental conditions(Flaounas et al. 2013). In this region,
the increasing risk of late frost events represents one of the major
threats associated with the future global change (Zohner et al.2020). Indeed, increasing spring temperatures has been observed
stimulating earlier leaf unfolding (Gordo & Sanz 2010; Allevatoet al. 2019), thus potentially exposing young leaves and shoots
to spring frost damage (Augspurger 2013), especially at high elevation
(Vitasse, Schneider, Rixen, Christen & Rebetez 2018). Depending on
species, temperatures below −4°C can destroy the “fresh” leaves and
shoots reducing − up to even blocking − the photosynthetic capacity of
trees for several weeks. In this case, the resource requirements for new
leaves formation, and trees life maintenance, must necessarily rely on
the remobilization of carbon (C) reserves (Dittmar, Fricke & Elling
2006; D’Andrea et al. 2019). Moreover, severity, duration, and
frequency of drought events have all been increasing in the last decades
(Spinoni, Naumann, Vogt & Barbosa 2015). European beech (Fagus
sylvatica L.), one of the most diffused native tree species in Europe,
is known to be drought sensitive (Bolte et al. 2016). Hence,
drought events can negatively affect physiological performance (Rezaieet al. 2018), carbon allocation (D’Andrea, Guidolotti, Scartazza,
De Angelis & Matteucci 2020a), reproductive capacity (Nussbaumeret al. 2020), as well as the growth and competitive strength of
the species (Peuke, Schraml, Hartung & Rennenberg 2002) which may all
impact its future distribution (Noce et al. 2017).
Growth and non-structural carbohydrates (NSCs; i.e. sucrose, fructose,
glucose and starch) dynamic are among the most strongly affected
ecosystem processes by spring frost and summer drought (Li et al.2018). An increasing body of evidence has shown that NSCs dynamic is not
a pure passive deposit and removal of C compounds, but it represents a
key process actively controlled by plants to finely regulate C
source-sink balance and to buffer the difference between C supply and
demand at different timescales (Scartazza et al. 2001; Sala,
Woodruff & Meinzer 2012; Carbone et al. 2013; Fatichi, Leuzinger
& Körner 2014; Martínez-Vilalta et al. 2016; Moscatello et
al. 2017; Collalti et al. 2020). Therefore, NSCs could play a
crucial role in counteracting the negative effects of extreme weather
events on beech forests, contributing to their resilience and survival
(Scartazza, Moscatello, Matteucci, Battistelli & Brugnoli 2013;
D’Andrea et al. 2019). However, despite the recognized importance
of NSCs for plant productivity and resilience, little is known regarding
their seasonal regulation and trade-off with growth and reproduction in
forest trees (Merganičová et al. 2019; Tixier et al.2020). The stemwood plays a key role in the aforementioned mechanisms,
because it, together with coarse roots, contains most of the NSCs of the
tree plants (Barbaroux, Bréda & Dufrêne 2003; Richardson et al.2015)
In this work, we studied the effects of spring late frost and summer
drought in a long-term research plot established in a Mediterranean
beech forest (Collelongo, Abruzzi Region, Italy). The site is located in
the large area of the Central-South Italy where in spring 2016, due to
unusually warm preceding weeks, leaf unfolding occurred up to 15-20 days
earlier than the normal average, followed by frost, that caused the
complete loss of the newly grown leaves. Moreover, in 2017, a strong
summer drought, due to a combination of drastic reduction of
precipitation associated with high air temperature in late July and
August, interested a huge area of the Mediterranean basin (Basciettoet al. , 2018; Nolè et al. , 2018; Allevato et al. ,
2019; Rita et al. , 2019), including the Collelongo site.
Notwithstanding these events were monitored through remote sensing
techniques, in situ evaluations of their effects on ecosystem
functionality are limited. Phenology, growth and stemwood NSCs dynamic
in the Collelongo beech stand were investigated during 2016 (i.e. the
year of the late frost event) and 2017 (i.e. the year of the summer
drought event) and compared to the historical intra-annual data
collected earlier at the site.
The objectives of the study were to: i ) quantify the magnitude of
the effects of such extreme events on ecosystem functioning; ii )
verify the role of NSCs in mediating source-sink balance following the
strong alteration of C activity; iii ) evaluate the interplay and
trade-off between carbon allocation to canopy, stem and C reserves. The
aim was to predict how these responses and regulation processes could
contribute to the resilience of beech to extreme weather events
associated with future global change.